2D Space Shooter Part 6: Enemies
This won't be a space shooter without enemies to shoot. Let's define our enemies. Again we will subclass our ship class for enemies.
Create a new file named 'enemy.lua'. This new file will depend on the Ship class too, so right click on the file name in gideros studio and access 'Code dependencies'. From there tell Gideros that enemy.lua will depend on ships.lua.
Enemy class
Enemies will come from the top of the screen, so ships will head toward the bottom.
EnemyShip=Core.class(Ship)
function EnemyShip:init(type,x,y)
self.posx=x
self.posy=y
self:setRotation(180)
self.fire=true
ENEMIES:addChild(self)
end
function EnemyShip:advance(amount)
self.posy+=amount
end
function EnemyShip:tick(delay)
self:setPosition(self.posx,self.posy)
Ship.tick(self,delay)
end
function EnemyShip:explode()
-- Here we could show some explosion animation and play a sound
self:destroy()
end
Level management
Now we need to make the enemies appear in waves. This part is a bit trickier, as it involves choosing a format to design our levels.
I decided to use a plain text representation: enemies will be placed in up to five positions on each row, possibly spanning multiple rows. So each line of our level definition will be six character long: one character for each possible position, indicating if a ship you be spawn at the location, and which kind of ship, plus a sixth character to control the ship waves: the sixth character with tell how much time to wait before the next row of ship shows up, or whether the row is the last of a wave.
In a new file called 'level.lua' we'll define our level manager, starting by our first (and only in this tutorial) level.
local LEVEL1=[[
..A..0
.....!
.A.A.0
.....!
..A..1
.A...1
...A.0
.....!
..A..0
.B.B.0
.....!
A...A0
..C..0
.....!
]]
local LEVELS={ LEVEL1 }
So this is our level data. We can almost see how ships will be shown on screen.
Now our loading (init) and sequencing (tick) code:
local SCROLL_DELTA=0.02
Level=Core.class(Object)
function Level:init(number)
-- Will hold our level info
self.levelinfo={}
-- Will hold our position in the level
self.levelline=1
-- Hold all the enemy ships in the current wave
self.enemies={}
-- Hold the enemy line scroll amount
self.scroll=0
-- Fill in level info
for level_line in LEVELS[number]:gmatch("[%w%p]+") do
assert(#level_line==6,"Level description line is not 6 characters long.")
local wait=tonumber(level_line:sub(6)) or 0
local clear=level_line:sub(6)=="!"
table.insert(self.levelinfo,{ enemies=level_line:sub(1,5), wait=wait, clear=clear})
end
--Register as actor
ACTORS[self]=true
end
function Level:tick(delay)
if self.scroll==0 then
--Not started scrolling: build an enemy row
local lineinfo=self.levelinfo[self.levelline]
for i=1,5 do
local type=lineinfo.enemies:sub(i,i)
if type~="." then
local enemy=EnemyShip.new(type,SHIP_SIZE*i+SCR_LEFT,SCR_TOP-SHIP_SIZE/2)
self.enemies[enemy]=true
enemy:advance(SCROLL_DELTA*SHIP_SIZE)
end
end
self.scroll=SCROLL_DELTA
elseif self.scroll<1 then
self.scroll+=SCROLL_DELTA
for k,_ in pairs(self.enemies) do
if ACTORS[k] then
k:advance(SCROLL_DELTA*SHIP_SIZE)
end
end
else
local lineinfo=self.levelinfo[self.levelline]
if lineinfo then --If no lineinfo, it means that we reached the end of the level
if lineinfo.wait>0 then
lineinfo.wait-=SCROLL_DELTA
else
local enemy_count=0
if lineinfo.clear then
for k,_ in pairs(self.enemies) do
if ACTORS[k] then enemy_count+=1 end
end
end
if enemy_count==0 then
self.levelline+=1
if lineinfo.clear then
self.enemies={}
end
if self.levelinfo[self.levelline] then
self.scroll=0
else
-- Level finished
print("FINISHED")
end
end
end
end
end
end
Kicking off our level
It is time to see what we just did: let's start our level from main.lua by adding a single line:
Level.new(1)